Cheryl Gillan, the Welsh secretary of state, said that the government would investigate the causes of the Gleision colliery disaster. Photograph: Tim Ireland/PA

The investigation into the tragedy at Gleision colliery could take months to complete as mine safety inspectors piece together the cause of the accident.

The devastating rush of water into the drift mine is thought to have occurred when the roof over the miners' heads collapsed, releasing a huge volume of water from an old working above.

The sequence of events that led to the mine becoming inundated is unclear, but may have resulted from blasting and removing coal, undersized pillars holding up the roof, or another structural failure. Had the mine deteriorated over time, parts of the roof may have weakened and suddenly collapsed under the pressure of water that had collected in a mine above.

The Welsh secretary, Cheryl Gillan, said on Friday the government would carry out "a full investigation into this terrible incident".

The inquiry will initially be led by the police in accordance with the work-related deaths protocol agreed between the police, the HSE, local authorities and the Crown Prosecution Service, with HSE support. At a later stage the investigation is expected to be handed over to the HSE, the same government agency charged with making sure that mines abide by stringent safety regulations.

A spokeswoman for the HSE said the inspectorate's records showed no enforcement notices on the Gleision colliery, which was due an inspection later this year.

The inquiry will focus on what information the management of the colliery had about existing and disused mine workings in the vicinity. The law requires managers to employ a surveyor whose job it is to provide maps of nearby workings so miners can work a safe distance from them.

The HSE has specific regulations designed to prevent inrushes of water into mines. These make it the duty of the mine owner and manager to obtain all available information about nearby workings from the Coal Authority, where mine abandonment plans are filed, and to ensure that water inrushes do not happen.

But records of old mines are patchy. The failure of private mining companies to file abandonment plans in the early 20th century go some way to explain why people still fall down unknown mines today.

The regulations state that mining should not be carried out within 45 metres in any direction, of a layer of rock containing water, or any disused workings that are not linked to mines. Nor are miners supposed to work in tunnels within 37 metres of any disused mine workings.

In many cases, managers can proceed with mining if they believe there is no risk of flooding and inform the HSE and the mine workers of their reasoning. "If you are the surveyor, you need to know where the workings are and what the likely risks are, and water is very clearly a risk," said Pat Carragher, general secretary at the British Association of Colliery Management. "If you are going to undercut any contiguous workings, the things that are going to concern you are the vertical distance from any seam that might have been worked above you and might have been waterlogged."